Communications system with priority scheme for reduced access delay

Abstract

A communications system is described having a central station and a number of remote units arranged for communication over a common communications channel. Each remote unit has a predetermined priority parameter and each remote unit has a processor element (30), a receiver (28), a channel monitoring circuit (26) for sensing when the channel is free and a transmitter (29). The processor element is arranged to identify from received data a priority parameter of a unit currently transmitting. A comparison is performed between the priority parameter of that unit and the predetermined priority parameter, to initiate a timer function (108) in response to the channel monitoring circuit when the channel monitoring circuit indicates that the channel has become free, and to reset the timer function if the channel monitor circuit indicates that the channel has become occupied. The timer function times a selectable period which is dependent on the comparison and the timer function causes activation of the transmitter when the timer function times the selectable period. The invention is applicable to, but not limited to use in supervisory control and acquisition-of-data (SCADA) systems.

We claim: 
     
       1. A communications system comprising at least one central station and a number of remote units arranged for communication over a common communications channel, each remote unit having a predetermined priority parameter and each remote unit comprising:   a processor element;   a receiver coupled to the processor element for receiving signals from the channel and providing received data to the processor element;   a channel monitoring circuit associated with the receiver for sensing when the channel is free and providing an indication to the processor element;   a transmitter responsive to the processor element for transmitting on the channel;   wherein the processor element is arranged to identify from the received data a priority parameter of a unit currently transmitting, to perform a comparison between the priority parameter of that unit and the predetermined priority parameter, to initiate a timer function in response to the channel monitoring circuit when the channel monitoring circuit indicates that the channel has become free, and to reset the timer function if the channel monitor circuit indicates that the channel has become occupied, where the timer function times a selectable period which is dependent on the comparison and the timer function causes activation of the transmitter when the timer function times the selectable period.   
     
     
       2. A method of operating a communications system having at least one central station and a plurality of remote units arranged for communication over a common communications channel, the method comprising the steps of, at a first remote unit: receiving signals from the channel;   identifying from the received data a priority parameter of a second remote unit currently transmitting;   performing a comparison between the priority parameter of the second remote unit and a predetermined priority parameter of the first remote unit;   initiating a timer function in response to the channel monitoring circuit when the channel monitoring circuit indicates that the channel has become free and timing a selectable period which is dependent on the comparison;   resetting the timer function if the channel monitor circuit indicates that the channel has become occupied, and   activating the transmitter when the timer function times the selectable period.   
     
     
       3. A method according to claim 2, wherein the step of performing the comparison comprises subtracting the priority parameter of the other unit from the priority parameter of the remote unit to provide a priority difference and the step of timing comprises timing a selected number of predetermined units of time following sensing of free channel by the channel monitoring circuit, where the selected number of units timed is one less than the priority difference. 
     
     
       4. A method according to claim 3 wherein one predetermined unit of time is approximately equal to the channel resolution time of the system. 
     
     
       5. A method according to claim 4, wherein each remote unit has a unique identification number, where the identification numbers of remote units are allocated in numerical sequence and where the identification number of a remote unit is used as its priority parameter. 
     
     
       6. A method according to claim 3, wherein each remote unit has a unique identification number, where the identification numbers of remote units are allocated in numerical sequence and where the identification number of a remote unit is used as its priority parameter. 
     
     
       7. A method according to claim 2, wherein each remote unit has a unique identification number, where the identification numbers of remote units are allocated in numerical sequence and where the identification number of a remote unit is used as its priority parameter. 
     
     
       8. A remote unit of a communications system having a number of remote units arranged for communication over a common communications channel, the remote unit having a predetermined priority parameter and comprising: a processor element;   a receiver coupled to the processor element for receiving signals from the channel and providing received data to the processor element;   a channel monitoring circuit associated with the receiver for sensing when the channel is free and providing an indication to the processor element;   a transmitter responsive to the processor element for transmitting on the channel;   wherein the processor element is arranged to identify from the received data a priority parameter of a unit currently transmitting, to perform a comparison between the priority parameter of that unit and the predetermined priority parameter, to initiate a timer function in response to the channel monitoring circuit when the channel monitoring circuit indicates that the channel has become free, and to reset the timer function if the channel monitor circuit indicates that the channel has become occupied, where the timer function times a selectable period which is dependent on the comparison and the timer function causes activation of the transmitter when the timer function times the selectable period.   
     
     
       9. A radio communications system comprising at least one central station and a number of remote units arranged for communication over a common communications channel, each remote unit having a predetermined parameter and comprising:   a receiver having a channel monitoring circuit for sensing when the channel is free;   a demodulator and decoder circuit for demodulating signals received and decoding and identifying a parameter of a remote unit transmitting on the channel at a given time;   a counting arrangement for subtracting the parameter of the currently transmitting unit from the predetermined parameter of the unit in question to give a delay number;   a timer arrangement for waiting that number of delay units less one following sensing of free channel by the channel monitoring circuit;   a reset arrangement for resetting the timer arrangement when the channel monitoring circuit senses that the channel is no longer free before the selected number of units are timed and   a transmitter for transmitting on the channel when the timer arrangement counts to the selectable number of delay units.